1. Field of the Invention
This invention generally relates to a driving system of a thermal print head for use in a thermal recording device which is widely used as an output device of various terminal machines such as printers, copiers and facsimile machines. More in particular, this invention relates to a driving system for driving a thermal print head including a plurality of heat-producing elements, which are selectively driven by an electrical image signal to "burn" heat-sensitive paper or tape selectively to form a visual image directly on the heat-sensitive paper or on a recording medium such as plain paper through the heat-sensitive tape.
2. Background of the Invention
Thermal recording devices are well known in the art. In a typical thermal recording device, a sheet of heat-sensitive paper is moved in contact with a thermal print head provided with a plurality of electrically resistive or heat-producing elements arranged in a line, and driving currents are selectively supplied to the resistive elements by a driving circuit connected thereto in accordance with an image signal. The resistive elements produce heat when driving currents are supplied thereto, and the heat thus produced is applied to the heat-sensitive paper to form "burn" or dark spots thereby forming a reproduced image in the form of dot matrix.
In such thermal recording devices, quality of recorded image and resolution are primarily determined by the temperature of each heat-producing element at the time or recording operation, and fluctuations in temperature would cause irregularities in density of resulting image. Under the circumstances, in order to obtain a recorded image of high quality, it is necessary to maintain the temperature of the heat-producing elements within a predetermined range during recording operation.
Various approaches have been proposed to carry out thermal recording operation so as to obtain a recorded image of high quality without irregularities in density. In one such approach, temperature in the vicinity of a thermal array comprised of heat-producing elements is detected, and the level of a driving current to be selectively supplied to the heat-producing elements is varied in accordance with the detected temperature. In another approach, the pulse width of the driving current or the current flowing time is controlled in accordance with the detected temperature. In a further approach, the frequency of the driving current is varied in accordance with the detected temperature. It has also been proposed to provide a driving current signal comprised of a plurality of current pulses of fixed pulse width for recording a single dot or picture element and to control the number of such current pulses so as to maintain the heat-producing elements at desired temperature during recording operation.
However, these prior art approaches are not always satisfactory even in the case of a binary or black-and-white recording system because the temperature of the heat-producing elements is also affected by the ambient thermal conditions, such as temperature of the ambient, and heat accumulation phenomenon. Furthermore, in the case of half-tone or gray-scale recording, since the density at each tone level must be reproduced faithfully as much as possible, temperature of the heat-producing elements during recording operation is required to be controlled much more stringently.
FIG. 1 is a graph showing a characteristic curve of recording density or density of an image formed on heat-sensitive paper by "burning" as a function of temperature of the heat-producing elements or current flowing time through the heat-producing elements. In the graph, t.sub.1 through t.sub.4 in the abscissa indicate current flowing time periods while current is supplied to the heat-producing elements and #1 through #4 in the ordinate indicate density levels of four different tones. In the case of FIG. 1, the time period during which a driving current of fixed magnitude is supplied to each heat-producing element is controlled to carry out half-tone recording at four different tone levels #1 through #4. For tone level #1, current flow time t.sub.1 is selected. Similarly, for tone levels #2, #3 and #4, current flow times t.sub.1 +t.sub.2, t.sub.1 +t.sub.2 +t.sub.3 and t.sub.1 +t.sub.2 +t.sub.3 +t.sub.4 are selected, respectively.
Accordingly, by controlling the current flowing period in four steps between t.sub.1 and t.sub.1 +t.sub.2 +t.sub.3 +t.sub.4, density of a recorded image may be changed in four levels #1 through #4. As is obvious from FIG. 1, since the curve is nonlinear, periods t.sub.1 through t.sub.4 are not equal in length when the total density level is equally divided into four parts.
FIG. 2 illustrates the case in which half-tone recording is carried out with four different tone levels using a direct-drive type thermal print head provided with a data buffer capable of storing data for a single line. In the timing chart of FIG. 2, there is shown a relation between transfer of data to be recorded and current pulse or current flowing time period. In the case when half-tone recording is to be carried out with the use of a direct-drive type thermal print head, as data for #1 tone, the data for all of the dots for tones ]1 through #4 are, for example, given the logic "1." Similarly, as data for #2 tone, the data for all of the dots for tones #2 through #4 are given "1", and as data for #3 tone, the data for all of the dots for tones #3 and #4 are given "1". Finally, as data for #4 tone, the data of only those dots corresponding to the density of #4 tone are given "1".
FIG. 3 is a graph showing, as an example, the relation between the current flow time and temperature of heat-producing elements when half-tone recording is carried out as illustrated in FIG. 2. In the case of a direct-drive type thermal print head, data are inputted serially, so that upon completion of inputting of data for a single line, a strobe signal is applied to control the supply of current to the heat-producing elements. In the case of carrying out half-tone recording operation in such a structure, transfer of data and supply of current are implemented for each tone level, and such an operation is repeated over the number of tones to complete recording of a single line.
In this manner, in the case of half-tone recording, since transfer of data and supply of current for a time period corresponding to associated tone are repetitively carried out for each of the four different tones #1 through #4, the temperature of the heat-producing elements vary as indicated in FIG. 3. That is, since current is not supplied during transfer of data, there exists a cooling period which corresponds to the data transfer period. This tends to deteriorate thermal efficiency of the heat-producing elements. As a result, it is necessary to increase a current flowing time period so as to compensate cooling during data transfer. It is also to be noted that the presence of data transfer period can be a cause of low recording speed.
Therefore, in the case of carrying out half-tone recording with the use of a direct-drive type thermal print head provided with a data buffer memory capable of storing data for a single line, the larger the number of tone levels, the longer the recording time. Moreover, because of the presence of a cooling period associated with data transfer, temperature rise of the heat-producing elements becomes discontinuous, which necessarily complicates control for compensating temperature fluctuations of the ambient and temperature rise due to heat accumulation, so that a faithful reproduction of half-tone image using a thermal print head becomes extremely difficult.